Device for receiving and/or emitting electromagnetic waves with radiation diversity

ABSTRACT

The present invention relates to a device for receiving and/or transmitting electromagnetic waves with radiation diversity. This device comprises, on a common substrate, at least one antenna of the slot type formed by a closed curve, known as a slot antenna, electromagnetically coupled to a first supply line, and an antenna radiating parallel to the substrate, positioned inside the slot antenna and connected to a second supply line, said first and second supply lines being connected via a switching means to means for exploiting the electromagnetic waves. The device can be applied, in particular, in the field of wireless transmissions.

The present invention relates to a device for receiving and/ortransmitting electromagnetic waves with radiation diversity which can beused in the field of wireless transmissions, notably in the case oftransmissions in closed or semi-closed environments such as domesticwireless networks, gymnasiums, television studios, show venues orsimilar places, but also in wireless communication systems requiring aminimal size for the antenna system such as in mobile telephones.

In the known high-bit-rate wireless transmission systems, the signalstransmitted by the transmitter reach the receiver via a plurality ofdifferent routes. When these are combined at the receiver, the phasedifferences between the various radio waves having followed pathways ofdifferent lengths give rise to an interference figure which can cause atendency to fade or a significant degradation of the signal. Moreover,the position of the tendency to fade changes over time, depending onchanges in the environment, such as the presence of new objects orpassing people. This tendency to fade, caused by the multiplicity ofpathways, can lead to a significant degradation both in the quality ofthe received signal and in the performance of the system.

In order to fight against this tendency to fade, the technique mostoften employed is a technique known as spatial diversity. This techniqueconsists notably of using a pair of antennas having a wide spatialcoverage, such as two antennas of the “patch” type, linked to aswitching unit. The two antennas are spaced out by a distance which mustbe greater than or equal to λ0/2, where λ0 is the wavelengthcorresponding to the operating frequency of the antenna. With this typeof antenna, it can be shown that the probability of having both antennasin a fading condition simultaneously is very low. Moreover, theswitching unit allows the branch connected to the antenna presenting thehighest signal level to be selected by examining the received signalusing a monitoring circuit. However, the main drawback with thissolution is that it is relatively voluminous since it requires a minimumspacing between the radiating antennas in order to ensure an adequatedecorrelation of the channel responses seen through each radiatingelement.

Various solutions have been proposed for reducing the size of theantenna system while still ensuring an adequate diversity. Somesolutions have been the object of several patent applications filed inthe name of THOMSON Multimedia Licensing S.A. They consist, notably, ofusing several antennas of the slot type supplied via line-slottransitions and comprising means allowing a diversity of radiation to beobtained, notably diodes allowing switching onto one or other of theantennas depending on the level of the received signal.

Furthermore, in the IEEE article, Vol. 49, No. 5 May 2001, entitled“Diversity antenna for external mounting on wireless handsets”, it hasalso been proposed, in the field of mobile telephones, to link a λ/4slot with a monopole to produce a diversity radiation system. However,the proposed system is a relatively complex, three-dimensionalstructure.

The aim of the present invention is therefore to propose a new solutionfor a device for receiving and/or transmitting electromagnetic waveswith radiation diversity having an extremely compact structure whilestill exhibiting radiation patterns with a very good complementarity. Italso provides a device for receiving and/or transmitting electromagneticwaves with radiation diversity having a relatively low cost ofmanufacture.

Consequently, the subject of the present invention is a device forreceiving and/or transmitting electromagnetic waves with radiationdiversity, characterized in that it comprises, on a common substrate, atleast one antenna of the slot type formed by a closed curve,electromagnetically coupled to a first supply line, and an antennaradiating parallel to the substrate such as a monopole, a helixoperating in transverse mode or similar, positioned inside the slotantenna and connected to a second supply line, said first and secondsupply lines being connected via a switching means to means forexploiting the electromagnetic waves.

The device for the reception and/or transmission of electromagneticwaves described above exploits the fact that antennas of the slot typeformed by a closed curve, hereinafter referred to as slot antennas, aswell as antennas of the monopolar or helical type operating intransverse mode exhibit virtually omnidirectional radiation patternswith minima situated, respectively, in the plane of the substrate forthe slot antenna and along the axis of the monopole or helix for theother antenna. Thus, switching from one antenna to the other allows thechannel response through the antenna to be modified and allows thesystem to thus benefit from a gain in diversity.

According to preferred embodiments, the first supply line is implementedin microstrip technology or in coplanar technology. Furthermore, thefirst supply line has a length between its end and the electromagneticcoupling point equal to kλm/4, where k is an odd integer and λm theguided wavelength on the supply line at the central operating frequencywith λm=λ0/{square root}{square root over (εr_(eff))}, where λ0 is thefree-space wavelength and εr_(eff) the effective permittivity of theline. The second supply line is implemented in microstrip technology orby a coaxial line. When the line is implemented in microstriptechnology, a connection is made at the slot antenna between the partthat is external and the part that is internal to the slot, thisconnection being formed, for example, by a conducting insert having awidth equal to around two to three times the width of the lineimplemented in microstrip technology, so as not to interfere with theoperation of the microstrip line providing the excitation. In addition,in order to minimize the interference within the slot of the slotantenna, owing to the presence of the conducting connection, thisconnection is situated in an electrical short-circuit plane for the slotwhich is therefore the plane where the microstrip line providing theexcitation of the monopole or helical antenna crosses the slot antenna.

According to preferred embodiments, the slot antenna is formed by anannular slot of circular shape or formed by a closed curve of perimeterequal to k′λs where k′ is an integer and λs is the wavelength in theslot at the operating frequency and/or by a slot of polygonal shape suchas a square or rectangle. According to another feature of the presentinvention, the device for receiving and/or transmitting electromagneticwaves with radiation diversity may comprise several slot antennasinterlocking with one another so as to widen the operating band or toallow multiband applications.

Other features and advantages of the present invention will becomeapparent upon reading the description of various embodiments presentedwith reference to the appended drawings, in which:

FIG. 1 is a schematic perspective view of a first embodiment of thepresent invention,

FIGS. 2 and 3 are respectively a cross-sectional and a top view of thefirst embodiment,

FIGS. 4 and 5 show perspective views of the radiation patterns of themonopole and of the slot antennas, respectively, for a device accordingto FIGS. 1 to 3,

FIG. 6 shows a curve plotting the S parameters in dB as a function offrequency between the various “ports” for a device according to FIGS. 1to 3,

FIG. 7 is a cross-sectional view of a second embodiment of the presentinvention,

FIG. 8 is an identical curve to that in FIG. 6 for the secondembodiment,

FIGS. 9 and 10 show the radiation patterns of the slot and of themonopole antennas, for a device according to FIG. 7.

In order to simplify the description, in the drawings the same elementscarry the same reference numbers.

As shown in FIGS. 1 to 3, the device for receiving and/or transmittingelectromagnetic waves consists essentially of a slot antenna 1 formed bya closed curve, more particularly an annular slot, and of an antenna 2radiating parallel to the plane of the slot, namely a monopole in theembodiment shown. The monopole 2 is positioned at the center of the slotantenna 1. More specifically, as shown in FIGS. 2 and 3, the device ofthe present invention comprises a substrate made from dielectricmaterial 3 whose top surface has been metallized. The annular slot 1 isfabricated by demetallization of the metallic layer 4 around a circle ofdiameter depending on the operating wavelength of the device, moreparticularly its perimeter is equal to k′λs where λs is the wavelengthin the slot at the operating frequency and k′ is an integer.

Furthermore, a circular opening 5 of diameter D is provided at thecenter of the annular slot. This opening receives the monopole 2 in itscentral part which also passes through the substrate 3. An annularmetallic mounting disk 5 is provided on the lower face of the substrate3 under the monopole 2. As shown more particularly in FIG. 3, theannular slot 1 is excited, according to the method described by Knorr,by a microstrip line 6 connected to the port 1. This microstrip line 6is fabricated on the lower face of the substrate. Between its free end6′ and the electromagnetic coupling point with the slot 2, it has alength Lm=kλm/4, where λm is the wavelength on the line and k is an oddinteger.

Similarly, in the embodiment shown, the monopole 2 is excited by amicrostrip line 7.

As shown in FIG. 3, in order to ensure continuity of the ground planefor the microstrip line 7 that excites the monopole 2, a connection ismade between the internal disk and the external ring forming the annularslot 1. This connection is made by means of a conducting insert 8 ofwidth w that is large enough (width equal to around 2 to 3 times thewidth of the printed line providing the excitation) so as not tointerfere with the operation of the microstrip line providing theexcitation. In order to minimize the interference at the annular slotfrom the presence of this metallic insert, the latter is located in aplane of electrical short-circuit for the slot, which will therefore bethe plane where the line providing the excitation of the monopolecrosses the annular slot.

As presented in FIGS. 4 and 5, the annular slot 1 and the monopole 2exhibit radiation patterns that are virtually omnidirectional andrelatively complementary in that the minima m are situated, for theannular slot, in the plane of the substrate (in this case, along theaxis ox) and, for the monopole, along the axis of the latter (in thiscase the axis oz). Thus, switching from one port to the other (by meansof a switching device that is well known to those skilled in the art,such as a switch, positioned between the supply lines 6 and 7 and thepart for processing the signal, controlled by a control signal such asthe signal level, the signal-to-noise ratio or similar) allows thechannel response through the antenna to be modified and allows thesystem to thus benefit from a gain in diversity. Accordingly, if thedominant received signal arrives along the ox axis, for example, whichwould imply that a weak signal is received through the access connectedto the slot, by switching to the access connected to the monopole, it isvery probable that a signal with a substantial level will be receivedgiven that the direction ox corresponds to a maximum in the monopolepattern. A symmetric argument can be applied to the case where thedominant signal arrives along the oz axis, for example in the case of amultistage communication.

In this case, the coupling between the annular slot 1 and the monopole 2remains weak given:

-   -   i) the complementarity of the radiation patterns (the directions        of the maxima of one are in the direction of the minima of the        other);    -   ii) the orthogonality of the fields emitted by the slot and the        monopole antennas.

Minimal mutual interference can thus be expected between the tworadiating elements even though they occupy almost the same physicalspace.

In order to ensure correct operation of a transmission/reception devicesuch as described above, the dimensions of the latter have beencompletely chosen for operation at the central frequency of around 5.8GHz then simulated using the HFSS simulation package from Ansoft. Withreference to the schematic drawings in FIG. 1 to 3, the antenna systemformed by an annular slot 1 and a monopole 2 has the followingdimensions:

-   -   R_(int)=6.4 mm (internal radius of the slot)    -   R_(ext)=6.8 mm (external radius of the slot)    -   W_(s)=0.4 mm (width of the slot, W_(s)=R_(ext)−R_(int))    -   W_(m1)=0.3 mm (width of the microstrip line supplying the slot)    -   l_(m1)=8.25 mm (length of the microstrip line supplying the slot        between the port 1 and the line/slot transition)    -   l_(m1)′=8.25 mm (length of the microstrip line supplying the        slot between the line/slot transition and the end of the line in        open circuit)    -   D=2 mm (diameter of the demetallization at the center of the        slot)    -   L=13.21 mm (length of the monopole)    -   □=30 mm (diameter of the ground plane)    -   □_(monopole)=1 mm (diameter of the metallic wire forming the        monopole)    -   W_(m2)=0.2 mm (width of the microstrip line supplying the        monopole)    -   l_(m2)=8.4 mm (length of the microstrip line supplying the        monopole between the port 2 and the line/slot transition)    -   l_(m2)′=8.8 mm    -   insert 1.2 mm long (or 3% of the slot length)    -   a metallic disk of diameter 2 mm is placed under the monopole        (this facilitates the soldering of the monopole to its supply        line)

The substrate used is made of Rogers 4003 with relative permittivity_(r)=3.38 and thickness h=0.81 mm.

FIG. 6 shows the simulation results of the reflection coefficients atthe input of the lines supplying the annular slot (S11) and the monopole(S22) as well as the coupling coefficient (S21) between the two ports 1and 2. A good matching of the two antennas can be observed as well as anisolation better than 19 dB between the two accesses despite the extremeproximity of the two radiating elements, namely the slot 1 and themonopole 2.

In this case, the radiation patterns obtained at the monopole andannular slot access, respectively, are those shown in FIGS. 4 and 5.Despite a slight distortion of the monopole pattern, it can be observedthat the antenna system operates as desired, in other words thereforewith virtually omnidirectional, complementary patterns with the minimaalong the oz axis for the monopole and along the ox axis for the annularslot.

According to a variant, shown in FIG. 7, the monopole is excited by acoaxial line connected at the port 2. In this variant 2, the excitationof the monopole is on the substrate ground plane 9 side. In this case,the ground plane 9 is formed on the lower surface of the substrate 3.The antenna consisting of the annular slot 1 is formed in this groundplane. The supply line formed by a microstrip line 6 is now implementedon the upper surface of the substrate, the excitation taking place as inthe previous embodiment. Simulations specific to this variant have beencarried out using the HFSS package from Ansoft, on a particularimplementation dimensioned as follows:

-   -   R_(int)=6.4 mm (internal radius of the slot)    -   R_(ext)=6.8 mm (external radius of the slot)    -   W_(s)=0.4 mm (width of the slot, W_(s) =R _(ext)−R_(int))    -   W_(m1)=0.3 mm (width of the microstrip line supplying the slot)    -   l_(m1)=8.25 mm (length of the microstrip line supplying the slot        between the port 1 and the line/slot transition)    -   l_(m1)′=8.25 mm (length of the microstrip line supplying the        slot between the line/slot transition and the end of the line in        open circuit)    -   D=2 mm (diameter of the demetallization at the center of the        slot)    -   L=12.4 mm (length of the monopole)    -   □=30 mm (diameter of the ground plane)    -   □_(monopole)=1 mm (diameter of the metallic wire forming the        monopole)

The substrate used is made of Rogers 4003 with relative permittivity_(r)=3.38 and thickness h=0.81 mm.

The matching at the two accesses as well as the isolation between thetwo ports are shown in FIG. 8. The curve S21 shows a good isolationwhile the curves S11 and S22 show a good matching at the operatingfrequency of 5.8 GHz. FIGS. 9 and 10 present the radiation patterns,respectively at the slot and monopole access, of the device for thetransmission and/or reception of electromagnetic waves described above.It can be observed that the excitation of the monopole by coaxial line,which has the advantage of avoiding the crossing of the excitation lineof the monopole and the slot antenna, presents a better isolation(isolation greater than 22 dB) than in the case of the excitation bymicrostrip line and the monopole pattern is no longer distorted. Thisadvantage is gained at the expense of an increase in complexity of theantenna structure (slot and monopole access on opposite faces of thesubstrate and of different types: coaxial line and microstrip line).

Further modifications may be included such as the use of a helixoperating in the transverse mode in place of the monopole, the use of adouble or multiple slot in order to widen the band or for multibandapplications, tangential supply of the slot in place of a Knorr-typesupply, and the deformation of the annular slot to further reduce itssize, where it could also take the form of a square, a rectangle orother polygon while still remaining within the scope of the definitiongiven above. Similarly, the monopole or helix may be replaced byantennas of the same type which can be placed at the center of the slotantenna and which radiate in a direction parallel to the substrate. Thesupply line of the slot antenna can be implemented as a line inmicrostrip technology or in coplanar technology. In addition, the slotantenna may have means, such as notches in the case of an annular slot,that allow it to operate in cross-polarization mode.

1. A device for receiving and/or transmitting electromagnetic waves withradiation diversity, comprising, on a common substrate, at least a firstantenna of slot type, the slot being in the form of a closed curve ofperimeter equal to k′λs where λs is the wavelength in the slot at theoperating frequency and k′ an integer, said first antenna beingelectromagnetically coupled to a first supply line, and a second antennaradiating in a direction parallel to the substrate said second antennabeing positioned inside the curve forming the first antenna and beingconnected to a second supply line, said first and second supply linesbeing connected via a switching means to means for exploiting theelectromagnetic waves.
 2. The device as claimed in claim 1, wherein thefirst supply line is implemented in microstrip technology or in coplanartechnology.
 3. The device as claimed in claim 2, wherein the firstsupply line has a length between its end and the electromagneticcoupling point equal to kλm/4, where k is an odd integer and λm theguided wavelength on the supply line at the central operating frequencywith λm=λ0/{square root}{square root over (εr_(eff))}, where λ0 is thefree-space wavelength and εr_(eff) the effective permittivity of theline.
 4. The device as claimed in claim 1, wherein the second supplyline is implemented in microstrip technology or by a coaxial line. 5.The device as claimed in claim 4, wherein when the second supply line isimplemented in microstrip technology, a connection is made at the slotantenna between the part that is external and a part that is internal tothe slot.
 6. The device as claimed in claim 5, , wherein the connectionis formed by a conducting insert having a width equal to 2 to 3 timesthe width of the line implemented in microstrip technology.
 7. Thedevice as claimed in claim 5, wherein the connection is positioned in anelectrical short-circuit plane for the slot.
 8. The device as claimed inclaim 1, wherein the first antenna of slot type is formed by an annularslot or a slot of polygonal shape such as a square or rectangle.
 9. Thedevice as claimed in claim 1, wherein the second antenna radiatingparallel to the substrate is formed by a monopole or a helix operatingin transverse mode.
 10. The device as claimed in claim 8, wherein itcomprises several antennas of slot type interlocking one with another.11. The device as claimed in claim 1, wherein the second antennaradiating parallel to the substrate is positioned at the center of theantenna or antennas of slot type.
 12. The device as claimed in claim 6,wherein the insert is positioned in an electrical short circuit planefor the slot.